Pin name/function (Newbie question)

Thread Starter

NY10

Joined Jul 30, 2010
47
Hi, all

I have a EP3CxE144 and some pin#'s indicate I/O.

As far as I understand, I/O means input and output. so does this mean the particular pin#'s do input and output functions at the same time. (or sometimes it does input and other time it does output functions)

or am I misunderstanding ?

I am going to attach the document, so that you can understand it better.

Thanks.
 

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tom66

Joined May 9, 2009
2,595
Yes they are known as tristate pins. They can be an input, an output high, or an output low, they have three states. Input mode is sometimes called high-Z or high-impedance mode, because little current flows in when in high-Z mode; it's almost as if it were disconnected from the circuit.
 

BMorse

Joined Sep 26, 2009
2,675
Yes they are known as tristate pins. They can be an input, an output high, or an output low, they have three states. Input mode is sometimes called high-Z or high-impedance mode, because little current flows in when in high-Z mode; it's almost as if it were disconnected from the circuit.

I beg to differ about you explanation for a tri state I/O pin:

This is what Wiki says it is:
In digital electronics three-state, tri-state, or 3-state logic allows output ports to assume a high impedance state in addition to the fundamental 0- and 1-levels, effectively "removing" the output from the circuit. This allows multiple circuits to share the same output line or lines (such as a bus).
Tri-State has nothing to do with the pin being an input....

B. Morse
 

kingdano

Joined Apr 14, 2010
377
BMorse is correct

I/O pins which can be "tri-stated" can be used as an input to the device, output from the device (either exclusively, or if you are advanced in your coding, coincidentally in the same program) or in a 3rd state (hence tri) which is high-Z (high impedance) such that if you are in a sensitive circuit you can block the additional load of the devices I/O pin if need be.

This is helpful for protecting the device on start-up as well.
 

tom66

Joined May 9, 2009
2,595
An input/output pin can be tristated. An output pin can also be tristated (you can buy buffers which do this.) But guess which has an input in high-Z mode? I'll give an example with PIC microcontrollers. The register files for example are named TRISA, TRISB. This terminology comes from TRIState port A, B, and so on. I was giving an example relevant to the OP's FPGA, as it does not have tristate output buffers.
 
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